This invention relates to a process for recovering a pure CO fraction from a charge containing hydrogen, carbon monoxide and methane from which water and carbon dioxide have optionally been removed and is particular to a process wherein the charge is scrubbed with a supercooled liquid methane-containing stream, the resultant hydrogen being stripped out, and the resultant CO/CH.sub.4 -rich fraction is being separated by rectification to produce a pure CO fraction and a CH.sub.4 -rich fraction. The requirements of the term "pure" CO depends on the use of the CO, but in general, pure CO contains not more than 2000 mol-ppm.
As the result of increased purity requirements imposed on gaseous separation products, the increasing importance of the operating costs of a facility, and the continuous improvement of available thermodynamic data, a continuous technical evolution has taken place in the separation of H.sub.2 /CO in recent years.
As in the past, steam reformer effluent is the main source of an H.sub.2 /CO charge fraction. But also the gasification of heavy oil with oxygen--therefore partial oxidation--assuming an inexpensive oxygen source, has become important in recent years for providing an H.sub.2 /CO supply.
Most of the carbon monoxide produced in this way is used in the production of formic and acetic acid. Another use is in polycarbonate chemistry, which requires high-purity phosgene which in turn requires extremely-high-purity carbon monoxide as a raw material. The methane content of the carbon monoxide here must be less than 10 mol-ppm, and the hydrogen content less than 1000 mol-ppm, preferably less than 100 mol-ppm. The hydrogen byproduct produced in the recovery of pure CO is used, optionally after subsequent fine purification, for a variety of hydrogenation purposes.
The articles of R. Fabian in LINDE Reports from Technik und Wissenschaft [Technology and Science] No. 55, 1984, pp. 38 to 42, and Dr. R. Berninger in LINDE Reports from Technik und Wissenschaft No. 62, 1988, pp. 18 to 23 provide a survey of the currently used processes for producing carbon monoxide with hydrogen as a byproduct. A process for the recovery of pure CO by means of methane scrubbing and integrated N.sub.2 /CO separation is depicted in FIG. 6 and described in the pertinent description of the last-mentioned LINDE Report. In this known process, the carbon monoxide, hydrogen and nitrogen are scrubbed out of the gaseous charge gas in a first scrubbing column by means of supercooled liquid methane. In a downstream hydrogen stripping column, hydrogen is removed from the fraction drawn off at the bottom of the scrubbing column. The H.sub.2 -free mixture drawn off at the bottom of this stripping column is then separated in another separating column into an N.sub.2 /CO fraction (top product) and a liquid CH.sub.4 fraction (bottoms product). This liquid CH.sub.4 fraction is pumped up to the raw gas pressure and delivered to the scrubbing column as scrubbing agent. In a fourth column, the H.sub.2 /CO fraction is separated into a CO product fraction and an N.sub.2 -rich fraction. If the gaseous charge does not contain any nitrogen, this fourth column can be omitted. The heated CO product fraction is compressed together with the so-called circulating CO and delivered at the required product pressure. A carbon monoxide cycle with an expansion turbine is used to supply the cold values for this process.
To scrub out the carbon monoxide and nitrogen as completely as possible in the scrubbing column, it is necessary that the methane used as scrubbing agent be delivered in as pure a form as possible to the scrubbing column. To achieve a CH.sub.4 purity as high as possible for the scrubbing agent, a high boiling power, i.e., a large amount of vapor, is necessary for mass transfer in that column in which the CH.sub.4 -rich fraction is recovered, thereby increasing the cost of the process.